Fuel assembly

ABSTRACT

Each of short-length fuel rods  3  is arranged at a position other than in 3×3 corner regions  6  to  9  in such a manner as not to be simultaneously adjacent to a water rod  5  and others of the short-sized fuel rods. Gd fuel rods  4  are arranged at positions excluding the outer periphery, and the number of those of the Gd fuel rods  4  adjacent to the short-length fuel rods  3  is one-half or less the total number of the Gd fuel rods. At a transverse cross-section of a region upward from upper ends of the short-sized fuel rods  3 , the amount of burnable poison contained in a polygonal region  10  whose vertexes are located at centers of those of the first fuel rods  3  arranged at the outermost layer is smaller than the amount of burnable poison outside the region  10 . With this configuration, a critical power can be improved in consideration of both a distribution of the flow of coolant and a distribution of a thermal power in the fuel assembly.

This application is continuation of 09/022678 filed Feb. 12, 1998 nowU.S. Pat. No. 6,061,416.

BACKGROUND OF THE INVENTION

The present invention relates to a fuel assembly to be loaded in aboiling water reactor, and particularly to a fuel assembly aimed atattainment of high burnup.

In a boiling water reactor, recently, high burnup for increasing theamount of energy generated per unit weight of fuel is expected from theviewpoint of both effective utilization of uranium resource andreduction in generation amount of spent fuel. To attain high burnup, itis necessary to enhance an average enrichment of a fissile contained ina fuel assembly. It is also known that, to promote effective reaction ofa fissile for keeping stability of a boiling water reactor, it isrequired to increase a rate of the amount of a moderator with increasedenrichment of a fissionable material.

A technique for effectively increase a rate of the amount of a moderatoris disclosed FIG. 24 of in Japanese Patent Laid-open No. Hei 5-232273.This technique describes a fuel assembly 1 shown in FIG. 2, which haslarge-sized water rods 5, and short-length fuel rods 3 shorter thanother fuel rods 2. In the fuel assembly shown in FIG. 2, the fuel rodsare arranged in a square grid patten of 9-columns/9-rows, and two piecesof the large-sized water rods 5 are arranged in a region containingseven grid points at a central portion of the fuel assembly. The numberof long-length fuel rods 2 is 66 pieces and the number of theshort-length fuel rods is 8 pieces. The use of the short-sized fuel rodsextends a flow passage of coolant on an upper portion of the fuelassembly, to increase the amount of a neutron moderator on an upperportion of a core, thereby promoting a fission reaction. The use of theshort-sized fuel rods has another effect of reducing, a pressure lossfrom the viewpoint of the flow of coolant, thus improving stability ofthe core.

A critical power of a fuel assembly is important as the scale of athermal margin of the fuel assembly to be loaded in a boiling waterreactor. The critical power is defined as a maximum thermal power of thefuel assembly within a range in which there occurs no boiling transitionof fuel rods under a condition of the amount of coolant flowing in thefuel assembly. The larger the critical power of the fuel assembly, thelarger the thermal margin. In many cases, since boiling transitionoccurs at a fuel rod exhibiting the maximum thermal power to restrictthe critical power of the fuel assembly. Accordingly, the critical powerbecomes larger as a difference in power between the fuel rods becomessmaller and a power distribution in the fuel assembly becomes flatter.On the other hand, even when the power of the fuel rods is uniform, ifthere exists unevenness in the flow rate of coolant around each fuelrod, boiling transition tends to occur at a position where the flow rateof coolant is small, thus making small the critical power.

With respect to a thermal margin of a fuel assembly includingshort-sized fuel rods, Japanese Patent Laid-open No. Hei 5-232273describes a technique in which short-sized fuel rods are provided atpositions other than in the vicinity of corners of a fuel assembly, toflatten a power distribution in the fuel assembly. Besides, JapanesePatent Laid-open No. Hei 5-341071 describes the fact that upper portionsof fuel rods adjacent to short-sized fuel rods are effectively cooledbecause a flow passage of coolant is wide on the upper side of upperends of the short-sized fuel rods.

With respect to the fuel assembly having short-sized fuel rods describedin Japanese Patent Laid-open No. Hei 5-232273, a configuration in whichfuel rods containing burnable poison are arranged for reducing a powerpeaking at a cool state is disclosed in Japanese Patent Laid-open No.Hei 8-292281.

Of the above-described prior art fuel assemblies, the fuel assemblydisclosed in Japanese Patent Laid-open No. Hei 5-232273 describes thatthe arrangement of the short-sized fuel rods at positions other than inthe vicinity of the corners improves stability while flattening thepower distribution in the fuel assembly; however, the document fails tosufficiently examine the effect on the flow of coolant.

In the fuel assembly disclosed in Japanese Patent Laid-open No. Hei5-232273, the arrangement of the short-sized fuel rods at positionsother than in the corner regions acts to increase the critical powerfrom the viewpoint of flattening of the power distribution. On the otherhand, according to the fuel assembly disclosed in Japanese PatentLaid-open No. Hei 5-341071, the arrangement of the short-sized fuel rodsat positions other than in the corner regions acts to decrease thecritical power from the viewpoint of flow of a coolant. As a result, thenet effect of improving the critical power is unclear.

Japanese Patent Laid-open No. Hei 5-341071 describes the effect due tothe local flow of coolant at positions of the short-sized fuel rods. Thedocument, however, does not examine an effect on the flow distributionof the entire fuel assembly including positions apart from theshort-sized fuel rods and an effect of the short-sized fuel rods on thepower distribution in the fuel assembly.

Japanese Patent Laid-open No. Hei 5-232273 does not examinedistributions of a fissile or burnable poison. On the other hand,Japanese Patent Laid-open No. Hei 8-292281 describes a distribution ofburnable poison. The document, however, is intended to suppress a powerpeaking at a cool temperature, and it does not examine improvement inthermal margin upon operation of the fuel assembly.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a fuel assemblycapable of improving a thermal margin, particularly, a critical powerupon operation in consideration of both a distribution of the flow ofcoolant and a distribution of a thermal power in the fuel assemblyhaving short-sized fuel rods. The distribution of thermal power isaffected by distributions of a fissile and burnable poison, andarrangement of short-sized fuel rods.

A second object of the present invention is to provide a fuel assemblycapable of improving a critical power while reducing a void coefficientto improve stability of a high burnup core.

To achieve the first object, according to a first invention, there isprovided a fuel assembly including: a plurality of fuel rods including aplurality of first fuel rods shorter in length than others of the fuelrods and a plurality of second fuel rods containing burnable poison, theplurality of fuel rods being arranged in a square grid pattern of9-columns/9-rows or more; and at least one neutron moderator rod ;wherein each of the first fuel rods is arranged at a position other thanin 3-columns/3-rows regions (3×3 corner regions) at four corner portionsin such a manner as not to be simultaneously adjacent to said at leastone neutron moderator rod and others of the first fuel rods in the fourdirections of front and rear and right and left; the second fuel rodsare arranged at positions excluding the outermost periphery; the numberof those, of the second fuel rods, adjacent to the first fuel rods insaid four directions of front and rear and right and left is one-half orless the total number of the second fuel rods; and at a transversecross-section of a region upward from upper ends of the first fuel rods,the amount of burnable poison contained in a polygonal region whosevertexes are located at centers of those, of the first fuel rods,arranged at the outermost layer is smaller than the amount of burnablepoison outside the polygonal region.

To achieve the first object, according to a second invention, there isprovided a fuel assembly including: a plurality of fuel rods including aplurality of first fuel rods shorter in length than others of the fuelrods and a plurality of second fuel rods containing burnable poison, theplurality of fuel rods being arranged in a square grid pattern of9-columns/9-rows or more; and at least one neutron moderator rod;wherein each of the first fuel rods is arranged at a position other thanin 3×3 corner regions at four corner portions in such a manner as not tobe simultaneously adjacent to said at least one neutron moderator rodand others of the first fuel rods in the four directions of front andrear and right and left; the second fuel rods are arranged at positionsexcluding the outermost periphery; and most of fuel rods being adjacentto the first fuel rods in said four directions of front and rear andright and left and containing no burnable poison have a maximum averageenrichment of a fissile in a region upward from upper ends of the firstfuel rods.

To achieve the second object, according to a third invention, there isprovided a fuel assembly including: a plurality of fuel rods including aplurality of first fuel rods shorter in length than others of the fuelrods and a plurality of second fuel rods containing burnable poison, theplurality of fuel rods being arranged in a square grid pattern of9-columns/9-rows or more; and at least one neutron moderator rod;wherein the first fuel rods are arranged at positions other than in 3×3corner regions at four corner portions, and the first fuel rods arearranged at each side of the outermost periphery; the second fuel rodsare arranged at positions excluding the outermost periphery; the numberof those, of the second fuel rods, adjacent to the first fuel rods inthe four directions of front and rear and right and left is one-half orless the total number of the second fuel rods; and at a transversecross-section of a region upward from upper ends of the first fuel rods,the amount of burnable poison contained in a polygonal region whosevertexes are located at centers of those, of the first fuel rods,arranged at the outermost layer is smaller than the amount of burnablepoison outside the polygonal region.

To achieve the second object, according to a fourth invention, there isprovided a fuel assembly including: a plurality of fuel rods including aplurality or first fuel rods shorter in length than others of the fuelrods and a plurality of second fuel rods containing burnable poison, theplurality of fuel rods being arranged in a square grid pattern of9-columns/9-rows or more; and a neutron moderator; wherein the firstfuel rods are arranged at positions other than in 3×3 corner regions atfour corner portions, and the first fuel rods are arranged at each sideof the outermost periphery; the second fuel rods are arranged atpositions excluding the outermost periphery; and most of fuel rods beingadjacent to the first fuel rods in the four directions of front and rearand right and left and containing no burnable poison have a maximumaverage enrichment of a fissile in a region upward from upper ends ofthe first fuel rods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transverse sectional view showing a first embodiment of afuel assembly of the present invention;

FIG. 2 is a transverse sectional view showing a prior art fuel assembly;

FIG. 3 is a transverse sectional view showing a fuel assembly of a firstcomparative example;

FIG. 4 is a transverse sectional view showing a fuel assembly of asecond comparative example;

FIG. 5 is a transverse sectional view showing a second embodiment of thefuel assembly of the present invention;

FIG. 6 is a transverse sectional view showing a third embodiment of thefuel assembly of the present invention;

FIG. 7 is a transverse sectional view showing a fourth embodiment of thefuel assembly of the present invention;

FIG. 8 is a transverse sectional view showing a fifth embodiment of thefuel assembly of the present invention;

FIG. 9 is a transverse sectional view showing a sixth embodiment of thefuel assembly of the present invention;

FIG. 10 is a transverse sectional view showing a prior art fuelassembly;

FIG. 11 is a transverse sectional view showing a seventh embodiment ofthe fuel assembly of the present invention;

FIG. 12 is a transverse sectional view showing an eighth embodiment ofthe fuel assembly of the present invention;

FIG. 13 is a transverse sectional view showing a ninth embodiment of thefuel assembly of the present invention;

FIG. 14 is a transverse sectional view showing a tenth embodiment of thefuel assembly of the present invention; and

FIG. 15 is a schematic vertical section showing the first embodiment ofthe fuel assembly of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment 1)

Hereinafter, a first embodiment in which the present invention isapplied to a fuel assembly for a boiling water reactor will be describedwith reference to FIGS. 1 and 15. FIG. 1 is a transverse sectional viewof the first embodiment, and FIG. 15 is a schematic vertical sectionalview of the first embodiment. This fuel assembly includes a squarecylinder type channel box 1 and a fuel bundle contained in the channelbox 1. The fuel bundle includes a plurality of fuel rods in each ofwhich fuel pellets containing a fissile are enclosed, an upper tie plate14 and a lower tie plate 15 for supporting upper and lower portions ofthe fuel rods respectively, and a plurality of spacers 16 for holdinggaps between the fuel rods.

The fuel rods include long-sized fuel rods each having a relatively longfuel effective length, and short-length fuel rods 3 each having a fueleffective length shorter than that of the long-sized fuel rod. Thelong-sized fuel rods include fuel rods 2, 11, 12 and 13 each containingonly uranium dioxide (UO₂), and burnable poison containing fuel rods 4each containing both UO₂ and gadolinia as burnable poison (hereinafter,referred to as “Gd fuel rods”). Each of the short-length fuel rods 3contains only UO₂. Reference numeral 5 indicates a large-sized waterrod, formed into a circular shape in cross-section, functioning as aneutron moderator.

With respect to an average uranium(U-235) enrichment (hereinafter,referred to simply as an “average enrichment”) of the long-sized fuelrods in a region upward from upper ends of the short-length fuel rods 3(hereinafter, referred to as an “upper region”), it becomes lower in theorder of the fuel rod 2, fuel rod 11 (character a in FIG. 1), fuel rod12 (character b in FIG. 1), and fuel rod 13 (character c in FIG. 1).That is, the fuel rod 2 has the maximum enrichment, and the fuel rod 13has the minimum enrichment. The fuel rods adjacent to the short-lengthfuel rods 3 in the four directions of front and rear and right and leftare all the fuel rods 2 each having the maximum enrichment. In addition,a state being adjacent to a fuel rod in the four directions of front andrear end right and left is referred to simply as “a state being adjacentto a fuel rod” hereinafter. The fuel rod 2 adjacent to the short-sizedfuel rod 3 at the outermost periphery of the fuel assembly is adjacentto one Gd fuel rod 4.

The upper end of the short-sized fuel rod 3 is at a level being about{fraction (17/24)} of a fuel effective length (length of a fuel packingportion) of the fuel rod 2. The upper end of a fuel packing portion ofthe short-sized fuel rod 3 is at a level being about {fraction (15/24)}of the fuel effective length of the fuel rod 2. The Gd fuel rod 4 has afuel effective length being substantially equal to that of the fuel rod2.

In this embodiment, the fuel rods are arranged in a square grid patternof nine-columns/nine-rows, and two pieces of the water rods 5 aredisposed in a region containing seven grid points at a central portionof the fuel assembly. Further, four pieces of the short-length fuel rods3 are disposed at each center of four outermost peripheral sides, andfour pieces of the remaining short-length fuel rods 3 are disposedadjacently to the water rods 5. The fuel rods of 66 pieces are presentat the transverse cross-section of the upper region of the fuelassembly. Any short-sized fuel rod 3 is not disposed inthree-columns/three-rows corner regions (hereinafter, referred to as“3×3 corner regions”) 6 to 9 at four corners of the fuel assembly.Further, the short-sized fuel rod 3 adjacent to the water rod 5 is notadjacent to another short-sized fuel rod.

The Gd fuel rod 4 contains burnable poison in the upper region. The Gdfuel rods 4 of 16 pieces are arranged at positions excluding theoutermost periphery of the fuel assembly. A region 10 is formed of apolygonal (square) shape whose vertexes are located at centers of fourpieces of the short-sized fuel rods arranged at the outermost layer ofthe fuel assembly. At the transverse cross-section of the upper regionof the fuel assembly, the amount of burnable poison contained in theregion 10 is equivalent to that contained in 6 pieces of the Gd fuelrods, which is smaller than the amount of burnable poison outside theregion 10 (equivalent to that contained in ten pieces of the Gd fuelrods).

In a core of a boiling water reactor, moderation of neutrons is promotedin a non-boiling water region which exists around a fuel assembly. Thisallows nuclear fission reaction to be actively generated from the fuelrods at the outermost periphery of the fuel assembly. As a result, thethermal power of the fuel rods at the outermost of the fuel assemblybecomes highest, tending to restrict the critical power of the fuelassembly. In particular, the 3×3 corner region is a region in which thepower tends to become highest in the fuel assembly.

In the prior art arrangement shown in FIG. 2, short-length fuel rods 3are present near corners of a fuel assembly. Accordingly, since amoderator (coolant) is concentrated at portions on the upper side of theshort-sized fuel rods 3, moderation of neutrons are further promoted, asa result of which the power of the fuel rods in the corner regionsbecomes higher.

In this embodiment, since any short-sized fuel rod is not present nearthe corners and thereby a moderator is not concentrated at the cornerregions, the power of the fuel rods at the corner regions is suppressed,to obtain a relatively flat power distribution. For example, for thesame distribution of uranium enrichment (hereinafter, referred to simplyas “enrichment”) in the fuel assembly, the power of the fuel rods at thesecond row from the corners in FIG. 1 is made to be about 5% smallerthan that of the fuel assembly shown in FIG. 2, with a result that, forthe same flow of the coolant, the critical power is improved.

On the other hand, as a result of evaluating the flow rate distributionof the coolant at the transverse cross-section of the upper region inFIG. 1, it becomes apparent that the flow rate of the coolant flowing inthe corner regions is made to be about 3% smaller than that of the fuelassembly shown in FIG. 2. This is because, in the case shown in FIG. 1,a large amount of the coolant flows in upper spaces of the short-lengthfuel rods 3 disposed at positions excluding the corner regions. Further,since the upper portions of the fuel rods 2 adjacent to the short-lengthfuel rods 3 face to a wide flow passage of the coolant, cooling iseffectively performed. From the viewpoint of the flow of the coolant,for the same power of the fuel rods at the corner regions, there is atendency that the critical power of the arrangement shown in FIG. 2becomes higher.

As described above, it becomes apparent that in the case of changingarrangement of the short-sized fuel rods from the arrangement shown inFIG. 2 to that shown in FIG. 1, there occurs both effects of increasingand decreasing the critical power. The present inventors havequantitatively evaluated the critical power and found the fact that thecritical power of a fuel assembly can be improved by arrangingshort-sized fuel rods not to be contained in 3×3 corner regions and alsoadjusting distributions of burnable poison and a fissionable material.

Further, as a result of fully examining arrangement of short-sized fuelrods, it becomes apparent that short-sized fuel rods should be arrangedin relation to neutron moderators (water rods). A neutron moderatorslightly generates heat, and in particular, since a flow passage ofcoolant becomes wider at a peripheral portion of a large-sized neutronmoderator disturbing a regular arrangement of fuel rods, the coolant isliable to flow in the peripheral portion of the neutron moderator.Consequently, the presence of a neutron moderator causes a deviation inflow rate of coolant in a fuel assembly.

FIG. 10 shows a transverse sectional view of a prior art fuel assemblydescribed in Japanese Patent Laid-open No. Hei 8-292281. In thisexample, short-length fuel rods 3 are arranged in a state beingconcentrated near water rods 5. As a result of evaluating flow ofcoolant in this arrangement, it becomes apparent that the flow rate ofthe coolant at corner regions is made to be about 3% lower than that inthe arrangement shown in FIG. 1. The reason for this is that the flow ofthe coolant is increased near the water rods. Accordingly, in this fuelassembly, the flow of the coolant is concentrated at a central portionbeing not strict in thermal condition, and the flow rate of the coolantis decreased at the corner regions being strict in thermal condition. Asa result, in this arrangement, the critical power is reduced.

In this embodiment, the enrichment of the short-length fuel rods 3 nearthe water rods 5 as shown in FIG. 10 is avoided. To be more specific, inthe arrangement shown in FIG. 1, each short-sized fuel rod 3 is disposednot to be simultaneously adjacent to the water rod 5 and anothershort-sized fuel rod 3. With this configuration, it is possible toachieve the first object of the present invention, that is, to obtain aneffect of improving the critical power of a fuel assembly.

Further, to achieve the second object of the present invention, that is,to improve a,void factor, it is known that arrangement of short-sizedfuel rods at the outermost periphery of a fuel assembly or near a waterrod is effective. On the other hand, it has been found that from theviewpoint of a distribution of the flow rate of coolant, it is desiredto arrange short-sized fuel rods on the outer peripheral side of a fuelassembly for increasing the flow rate of the coolant on the outerperipheral side. For this reason, in this embodiment, four short-sizedfuel rods 3 are disposed at each side of the outer most periphery of thefuel assembly. Such a configuration is larger in the effect ofmoderating neutrons than that shown in FIG. 2. As a result, an absolutevalue or a void coefficient is made small, tending to ensure stabilityof a high burnup core.

The objects of the present invention can be achieved by adoptingarrangement of short-sized fuel rods in consideration of a distributionof the flow rate of coolant in a fuel assembly as described above, andalso adopting distributions of burnable poison and a fissile which willbe described below.

A first comparative example for comparatively examining the effect ofthe Gd fuel rods is shown in FIG. 3. In this comparative example, thearrangement of the short-length fuel rods 3 is the same as that shown inFIG. 1, but the arrangement of the Gd fuel rods 4 is different from thatshown in FIG. 1. In the fuel assembly shown in FIG. 1, the amount ofgadolinia present in the 3×3 corner regions 6 to 9 in which theshort-length fuel rods 3 are not present is larger than that in the 3×3corner regions 6 to 9 of the fuel assembly shown in FIG. 3. Since lowenergy neutrons causing a fission reaction are absorbed by gadolinia,the power of the fuel rods in this region shown in FIG. 1 is made about3% smaller than that in the region shown in FIG. 3. That is, thecritical power of the fuel assembly in FIG. I is improved about 3% ascompared with the fuel assembly shown in FIG. 3.

Based on the above knowledge, the short-sized fuel rods are arranged tobe offset inwardly of the fuel assembly, and the Gd fuel rods (burnablepoison) are arranged to be offset outwardly of the fuel assembly. Now,there is assumed a polygonal region whose vertexes are located at thecenters of the short-sized fuel rods arranged at the outermost layer inthe fuel assembly. A relatively large amount of the coolant flows insidethis region, and the flow rate of the coolant is relatively lackingoutside this region. Accordingly, by arranging a relatively large amountof burnable poison outside this region, it becomes possible to reducethe power of the fuel rods and hence to cope with the lacking state ofthe flow rate of the coolant.

In this embodiment, since the short-sized fuel rods are not arranged inthe 3×3 corner regions, the Gd fuel rods can be concentratedly arrangedin the 3×3 corner regions. The flattening effect of the powerdistribution due to arrangement of the Gd fuel rods in the 3×3 cornerregions overcomes the demerit in terms of the above-described flow ratedistribution, so that the critical power of the fuel assembly in FIG. 1is improved more than that of the fuel assembly in FIG. 2. Further,since the possibility of causing boiling transition is small at aposition adjacent to the short-sized fuel rod, the fuel rod containingno burnable poison is desirably disposed at such a position, and the Gdfuel rod is disposed at a position being strict in thermal condition.This makes it possible to further improve the thermal margin.

In this embodiment, the possibility of causing boiling transition ishigh at the fuel rods at the outermost periphery in each of the 3×3corner regions, particularly, at the fuel rod 12 positioned at thesecond column from the corner. Based on such acknowledge, the Gd fuelrod 4 is disposed at a position inwardly adjacent to the fuel rod 12.Since the thermal power of the Gd fuel rod 4 is low, a thermal load to aperipheral flow passage becomes small as compared with the case wherethe fuel rod containing no burnable poison is adjacent to the fuel rod12, to thereby obtain a high cooling effect. Further, arrangement of theGd fuel rods in the region in which the short-sized fuel rods are notpresent is not only effective to lower the entire power of the fuel rodsin this region but also effective to prevent occurrence of boilingtransition of the adjacent fuel rods.

Incidentally, in the case where the Gd fuel rods are arranged at theoutermost periphery of the fuel assembly, the reactivity worth of acontrol rod at the beginning of burning tends to be reduced. In thisembodiment, the reactivity worth of a control rod is prevented frombeing reduced by arranging the Gd fuel rods 4 at positions other than atthe outermost periphery of the fuel assembly.

FIG. 4 shows a second comparative example for comparatively examining aneffect of the adjacent relationship between the short-sized fuel rod andthe fuel rod having the maximum enrichment. The fuel assembly shown inFIG. 4 is configured that in the fuel assembly shown in FIG. 1, each ofthe fuel rods 2 adjacent to the short-length fuel rods 3 at theoutermost periphery is replaced with each of the fuel rods 11 at thethird columns from the corners at the outermost periphery. In thiscomparative example, each of the fuel rods 2 (having the maximumenrichment) at the third columns from the corners at the outermostperiphery generates the critical power, to thereby restrict the criticalpower. On the contrary, in the fuel assembly shown in FIG. 1, the powerof each of the fuel rods at the third columns from the corners at theoutermost periphery is reduced about 4% as compared with that in thefuel assembly shown in FIG. 4.

In the fuel assembly shown in FIG. 1, the power of each of the fuel rodsat the fourth columns from the corners at the outermost periphery isincreased about 3% as compared with that in the fuel assembly shown inFIG. 4; however, since a large amount of the coolant is present on theupper side of the short-sized fuel rod 3 adjacent thereto, the fuel rod2 does not function as the fuel rod restricting the critical power.Also, in the fuel assembly shown in FIG. 1, since each of the fuel rods2 at the fourth columns from the corners is adjacent to the Gd fuel rod4 being relatively small in heat generation amount, the fuel rod 2 isless susceptible to boiling transition.

In general, the power of each of the fuel rods in the 3×3 corner regionstends to be increased, so that it is difficult to maximize theenrichment of the fuel rod. In this embodiment, since the short-lengthfuel rods 3 are not arranged in the 3×3 corner regions, most of the fuelrods adjacent to the short-length fuel rods 3 are not in the 3×3 cornerregions. Accordingly, in this embodiment, it is possible to maximize theenrichment of each of the fuel rods adjacent to the short-sized fuelrods 3.

To be more specific, in a condition in which an average enrichment of afuel assembly is specified, by maximizing an average enrichment in theupper region of fuel rods adjacent to short-sized fuel rods and locatedat positions being not severe in heat transfer just as this embodiment,an average enrichment of fuel rods located at other position beingstrict in heat transfer can be lowered. The flattening effect of thepower distribution due to the above arrangement in terms of averageenrichment overcomes the demerit of the above-described flow ratedistribution, so that the critical power of the fuel assembly shown inFIG. 1 can be improved more than that in the fuel assembly shown in FIG.2.

Also, in this embodiment, the above-described effect can be obtained bymaking lower the average enrichment in the upper region of the fuel rods2 arranged at a layer positioned inwardly from the outermost peripherythan that of the fuel rods 2 arranged at the outermost periphery.

Further, in the case where the number of the fuel rods at the transversecross-section of the upper region of the fuel assembly is decreased, aheat generation amount per one fuel rod in the upper region isincreased, tending to lower the critical power. Accordingly, to obtain acritical power higher than that of the fuel assembly shown in FIG. 2,the number of the fuel rods at the transverse cross-section of the upperregion is desired to be set at 66 pieces or more like this embodiment.

(Embodiment 2)

Next, a second embodiment of the present invention will be describedwith reference to FIG. 5. This embodiment is configured that in thefirst embodiment shown in FIG. 1, four pieces of the short-length fuelrods 3 adjacent to the water rods 5 are replaced with the fuel rods 2.The upper end of the short-sized fuel rod 3 is at a level being aboutone-half of the fuel effective length of the fuel rod 2. The fueleffective length of the Gd fuel rod 4 is at a level being about{fraction (71/72)} of the fuel effective length of the fuel rod 2. Atthe transverse cross-section of the fuel assembly in the upper region(upward from upper ends of the short-sized fuel rods 3) are present 70pieces of the fuel rods. The other configuration is the same as that ofthe first embodiment.

In this embodiment, a polygonal region whose vertexes are located atcenters of the short-length fuel rods 3 arranged at the outermost layeris the same as the region 10 in the first embodiment. The distributionof burnable poison at the transverse cross-section of the upper regionis also the same as that shown in FIG. 1.

In this embodiment, since the number of the short-length fuel rods 3 issmaller than that in the fuel assembly shown in FIG. 1, a distributionof the flow rate of coolant at the upper portion of the fuel assemblybecomes more uniform and the overall heat transfer area of the fuel rodsis increased about 2% as compared with that in the fuel assembly shownin FIG. 1. As a result, the critical power in this embodiment isincreased about 1% as compared with that of the fuel assembly shown inFIG. 1.

By setting the fuel effective length of the Gd fuel rod 4 to be slightlyshorter (about {fraction (1/72)}) than that of the fuel rod, the volumeof plenum in the Gd fuel rod 4 is increased. Since the plenum functionsas a space for storing gas discharged from fuel pellets, the structurein this embodiment is more suitable for high burnup. Such a slightdifference in fuel effective length little influences the effect of thepresent invention.

In addition, to obtain a high neutron absorbing effect by the Gd fuelrods 4, the Gd fuel rods 4 are desired to be disposed in such a manneras not to be adjacent to each other in the four directions of front andrear and right and left. From this viewpoint, in addition to arrangementof the Gd fuel rods 4 shown in FIG. 5, the Gd fuel rods 4 can bearranged at six position adjacent to the water rods 5 and obliquelyadjacent to the Gd fuel rods 4 in FIG. 5.

Consequently, a desirable neutron absorbing effect can be obtained byarranging the Gd fuel rods 4 at positions selected from 22 positionsincluding the above six positions.

(Embodiment 3)

A third embodiment of the present invention will be described below withreference to FIG. 6. Even in this embodiment, the fuel rods are arrangedin a square grid pattern of 9-columns/9-rows, and two pieces of thewater rods 5 are arranged in a region containing seven grid points at acentral portion of the fuel assembly. Four pieces of the short-lengthfuel rods 3 are disposed at the centers of the outermost periphery andtwo pieces of the remaining short-length fuel rods 3 are disposed atpositions adjacent to both the water rods 5. At the transversecross-section of the upper region of the fuel assembly are present 68pieces of the fuel rods. Even in this embodiment, the short-length fuelrods 3 are not disposed in the four 3×3 corner regions 6 to 9. Further,one short-sized fuel rod 3 adjacent to both the water rods 5 is notadjacent to the other short-sized fuel rod 3.

In this embodiment, 12 pieces of the Gd fuel rods 4 are all arranged atpositions in the 3×3 corner regions 6 to 9 excluding the outermostperipheries thereof, and are not adjacent to the short-sized fuel rods3. Further, a polygonal region whose vertexes are located at the centersof the short-length fuel rods 3 arranged at the outermost layer is thesame as the region 10 shown in FIG. 1. The Gd fuel rods 4 are allarranged outside the region 10.

In this embodiment, by arranging all of the Gd fuel rods 4 in the 3×3corner regions 6 to 9 not containing the short-sized fuel rods 3, it ispossible to effectively reduce the power of each corner region being lowin the flow rate of coolant and hence to improve the critical power.Further, in the fuel assembly shown in FIG. 6, the number of the Gd fuelrods 4 being low in heat generation amount is smaller than that in thefuel assembly shown in FIG. 1. Accordingly, in this embodiment, thepower distribution in the fuel assembly can be further flattened ascompared with that in the first embodiment, with a result that thecritical power is made higher.

(Embodiment 4)

A fourth embodiment of the present invention will be described belowwith reference to FIG. 7. In this embodiment, the fuel rods are arrangedin a square grid pattern of 9-columns/9-rows. A larger-sized water rod 5formed into a square shape in transverse cross-section is used as aneutron moderating rod. The water rod 5 is disposed at a regioncontaining nine grid points at a central portion of the fuel assembly.Four pieces of the short-length fuel rods 3 are disposed at the centersof the four outermost peripheral sides. That is, the short-length fuelrods 3 are not adjacent to the water rod 5 and are disposed at positionsother than in the four 3×3 corner regions.

In this embodiment, 68 pieces of the fuel rods are present at thetransverse cross-section of the upper region. The Gd fuel rods 4 of 16pieces are disposed at positions excluding the outermost periphery, andare not adjacent to the short-sized fuel rods 3. A polygonal regionwhose vertexes are located at the centers of the short-length fuel rods3 arranged at the outermost layer is the same as the region 10 shown inFIG. 1. The amount of burnable poison present in the region 10 isequivalent to that contained in six pieces of the Gd fuel rods, which issmaller than the amount of burnable poison outside the region 10(equivalent to that contained in 10 pieces of the Gd fuel rods).

Even in this embodiment having the square water rod, it is possible toobtain an effect similar to that in the first embodiment shown in FIG.1.

(Embodiment 5)

A fifth embodiment of the present invention will be described below withreference to FIG. 8. This embodiment is configured that in the fourthembodiment, four pieces of the short-length fuel rods 3 at the outermostperiphery are replaced with four pieces of the fuel rods 2 adjacent tothe water rod 5. The other configuration is the same as that of thefourth embodiment. At the transverse cross-section of the upper regionare present 68 pieces of the fuel rods. The Gd fuel rods 4 are alldisposed outside the region 10 having a polygonal shape whose vertexesare located at the centers of the short-length fuel rods 3 arranged atthe outermost layer, and are not adjacent to the short-sized rods 3.

In this embodiment, coolant is allowed to easily flow near theshort-length fuel rods 3 adjacent to the water rods 5, so that thereoccurs a deviation in flow of the coolant. In this embodiment, however,since each of the short-length fuel rods 3 adjacent to the water rod 5is not adjacent to another short-sized fuel rod 3, it is possible tosuppress the deviation in flow of the coolant at a small value.

Further, in this embodiment, the short-length fuel rods 3 and the waterrod 5, which promote moderation of neutrons in the surroundings, areconcentrated at the central portion, and the Gd fuel rods 4 which absorblow energy neutrons in the surroundings are arranged on the outerperipheral side. Thus, the distribution of the low energy neutrons isflattened, and also the distribution of the thermal power is flattened.As a result, although the critical power is equal to or less than thatin the fuel assembly shown in FIG. 7, the maximum linear power heatgeneration ratio is made smaller than that in the fuel assembly shown inFIG. 7. This leads to reduction in discharged amount of gas as afissionable product in the fuel rods.

(Embodiment 6)

A sixth embodiment of the present invention will be described below withreference to FIG. 9. In this embodiment, the fuel rods are arranged in asquare grid pattern of ten-columns/ten-rows. Two pieces of thelarge-sized water rods 5, each being formed into a circular shape intransverse cross-section, are disposed at a region containing eight gridpoints at a central portion of the fuel assembly. Two pieces of theshort-length fuel rods 3 are disposed at central positions each beingadjacent to both the water rods 5, and eight pieces of the short-lengthfuel rods 3 are provided in such a manner that two pieces are disposedat a central portion of each of the four outermost peripheral sides. Oneshort-sized fuel rod 3 adjacent to the water rods 5 is not adjacent tothe other short-sized fuel rod 3. Even in this embodiment, theshort-length fuel rods 3 are not arranged in four 3×3 corner regions 6to 9. At the transverse cross-section of the upper region are present 82pieces of the fuel rods.

The Gd fuel rods of 20 pieces are arranged at positions excluding theoutermost periphery of the fuel assembly, of which 8 pieces are arrangedin the 3×3 corner regions 6 to 9. The enrichment of burnable poisoncontained in a Gd fuel rod 4 a disposed at the second layer from eachcorner is higher than each of the other Gd fuel rods 4.

A polygonal region whose vertexes are located at the centers of theshort-length fuel rods 3 arranged at the outermost layer is an octagonalregion 10 shown in FIG. 9. The number of the Gd fuel rods contained inthe region 10 is 10 pieces, which is one-half the total number (20pieces) However, since the amount of burnable poison contained in the Gdfuel rod 4 a is smaller than the amount of burnable poison contained ineach of the other Gd fuel rods 4, the total amount of burnable poisoninside the region 10 is smaller than the total amount of burnable poisonoutside the region 10. As the enrichment of burnable poison is higher,the effect of absorbing neutrons becomes stronger and an absorbingability can be kept for a long burning period.

Like this embodiment, by making smaller the amount of burnable poisoninside the region 10 than the amount of burnable poison outside theregion 10 through adjustment of the enrichment of burnable poison, therecan be obtained an effect of improving a thermal margin. Further, sincein this embodiment, the number of the fuel rods is larger than that inthe first embodiment, the thermal margin can be further increased.

(Embodiment 7)

A seventh embodiment of the present invention will be described withreference to FIG. 11. In this embodiment, the fuel rods are arranged ina square grid pattern of 10-columns/ten-rows, and one large-sized waterrod 5, formed into a square shape in transverse cross-section, isdisposed in a region containing nine grid points at a central portion ofthe fuel assembly. Two pieces of the short-length fuel rods 3 arearranged in the central portion at positions adjacent to the water rod5, and eight pieces of the short-length fuel rods 3 are provided in sucha manner that two pieces are disposed at each of the four outermostperipheral sides. One short-sized fuel rod 3 adjacent to the water rod 5is not adjacent to the other short-sized fuel rod 3. Even in thisembodiment, the short-length fuel rods 3 are not arranged in the four3×3 corner regions. At the transverse cross-section of the upper regionare present 81 pieces of the fuel rods.

The Gd fuel rods 4 of 21 pieces are arranged at positions excluding theoutermost periphery of the fuel assembly. The number of the Gd fuel rodsadjacent to the short-length fuel rods 3 is two pieces, which is lessthan one-half the total number. A polygonal region whose vertexes arelocated at the centers of the short-length fuel rods 3 arranged at theoutermost layer is the same as the region 10 shown in FIG. 9. The amountof burnable poison contained in the region 10 is equivalent to thatcontained in nine pieces of the Gd fuel rods, which is smaller than theamount of burnable poison outside the region 10 (equivalent to thatcontained in 12 pieces of the Gd fuel rods).

In the case where there is a deviation in the distribution of the waterrod 5 and the short-length fuel rods 3 like this embodiment, thereeasily occur deviations in the power distribution and the distributionof the flow rate of coolant; however, the thermal margin can be improvedby adopting the arrangement of the Gd fuel rods 4 as shown in thisembodiment.

(Embodiment 8)

An eighth embodiment of the present invention will be described belowwith reference to FIG. 12. In this embodiment, the arrangement of the Gdfuel rods in the first embodiment (FIG. 1) is combined with thearrangement of the short-sized fuel rods in the third embodiment (FIG.6). In this embodiment, the fuel rods are arranged in a square gridpattern of nine-columns/nine-rows, and two water rods 5 are arranged ina region containing seven grid points at a central portion of the fuelassembly. Four pieces of the short-length fuel rods 3 are provided insuch a manner that one piece is disposed at each of the four outermostperipheral sides, and two pieces of the remaining short-length fuel rods3 are arranged at positions adjacent to the water rod 5.

One short-sized fuel rod 3 adjacent to the water rods 5 is not adjacentto the other short-sized fuel rod 3. At the transverse cross-section ofthe upper region are present 68 pieces of the fuel rods. Theshort-length fuel rods 3 are not arranged in the four 3×3 corner regions6 to 9. The upper end of the short-sized fuel rod 3 is at a level beingabout one-half the fuel effective length of the fuel rod 2.

The Gd fuel rods 4 of 16 pieces, each containing burnable poison in theupper region, are arranged at positions excluding the outermostperiphery of the fuel assembly. A polygonal region whose vertexes arelocated at the centers of the short-length fuel rods 3 arranged at theoutermost layer is the same as the region 10 shown in FIG. 1. At thetransverse cross-section of the upper region, the amount of burnablepoison contained in the region 10 is equivalent to that contained in sixpieces of the Gd fuel rods, which is smaller than the amount of burnablepoison outside the region 10 (equivalent to that contained in ten piecesof the Gd fuel rods).

In this embodiment, since the number of the short-sized fuel rods at thecentral portion is smaller than that in the first embodiment, the flowrate of coolant at the corner regions, and thereby the critical power ismade higher. On the other hand, since the number of the Gd fuel rods islarger than that in the third embodiment, the reactivity of the fuelassembly is made lower, and thereby the thermal power of the fuelassembly in the core can be made smaller.

(Embodiment 9)

A ninth embodiment of the present invention will be described below withreference to FIG. 13. In this embodiment, only the arrangement of the Gdfuel rods 4 in the eighth embodiment is changed. The Gd fuel rods inthis embodiment are arranged at each corner of the second layer from theoutermost periphery of the fuel assembly, positions adjacent thereto,and at each corner from the outermost periphery.

At the transverse cross-section of the upper region, the amount ofburnable poison contained in the region 10 is equivalent to thatcontained in two pieces of the Gd fuel rods, which is smaller than theamount of burnable poison outside the region (equivalent to thatcontained in 14 pieces of the Gd fuel rods).

In this embodiment, since burnable poison is concentratedly arranged inthe corner regions, it is possible to increase the effect of loweringthe power of the fuel rods at the peripheral portion. As a result, thepower of the fuel rods in the corner regions is reduced about 2% ascompared with that in the eighth embodiment shown in FIG. 12, andthereby the critical power is made higher about 2% or more.

(Embodiment 10)

A tenth embodiment of the present invention will be described below withreference to FIG. 14. In this embodiment, the number and the arrangementof the Gd fuel rods 4 in the eight embodiment are changed. The fuelassembly in this embodiment is that to be loaded in a core (D-latticecore) in which a width of a water gap between fuel assemblies isdifferent between a side in which a cross-type control rod is inserted(hereinafter, referred to as a “control rod side”) and the opposed side(hereinafter, referred to as an “anti-control rod side”).

In FIG. 14, the left upper side is the control rod side, and the rightlower side is the anti-control rod side. The width of the water gap iswider on the control rod side and narrower on the anti-control rod side.Since the effect of moderating neutrons becomes higher on the side wherethe width of the water gap is wide, the power of the fuel rods facing tothe wide water gap becomes relatively higher. Accordingly, in thisembodiment, the distribution of the Gd fuel rods is made asymmetric, torelax a deviation in power distribution.

To be more specific, while the Gd fuel rods 4 of three pieces arearranged in the 3×3 corner region 6 on the control rod side, no Gd fuelrod 4 is arranged in the 3×3 corner region 7 on the anti-control rodside. The Gd fuel rods 4 of 15 pieces are arranged at positionsexcluding the outermost periphery of the fuel assembly. At thetransverse cross-section of the upper region, the a mount of burnablepoison contained in the region 10 is equivalent to that contained infive pieces of the Gd fuel rods, which is smaller than the amount ofburnable poison outside the region 10 (equivalent to that contained inten pieces of the Gd fuel rods)

In this embodiment, the short-length fuel rods 3 are not present in the3×3 corner region 6 in which the power of the fuel rods are liable to bemaximized and the Gd fuel rods 4 are concentratedly arranged in theregion 6, so that the power distribution in the fuel assembly can beflattened. As a result, it is possible to enhance the critical power inthe fuel assembly loaded in the D-lattice core.

Although in the above embodiments, description is made using uranium asa fissionable material, the present invention can be applied to the caseusing a mixed oxide (MOX) of uranium and plutonium. That is, the effectof improving the thermal margin can be obtained for the case using theMOX fuel, like the case of using the uranium fuel, insofar as thepresent invention is applied to the distribution of both burnable poisonand a fissionable material.

What is claimed is:
 1. A fuel assembly comprising: a plurality of fuelrods including a plurality of short-length fuel rods shorter in lengththan others of said fuel rods and a plurality of long-length fuel rods,said long-length fuel rods having burnable poison rods with burnablepoison and fissionable material and fissionable material rods with noburnable poison and fissionable material, said plurality of fuel rodsbeing arranged in a square grid pattern of 9-columns/9-rows or more; andat least one neutron moderator rod; wherein each of said short-lengthfuel rods is arranged at a position other than in 3-columns/3-rowsregions at four corner portions in such a manner as not to besimultaneously adjacent to said at least one neutron moderator rod andothers of said short-length fuel rods in the four directions of frontand rear and right and left; said burnable poison rods are arranged atpositions excluding the outermost periphery; the number of those, ofsaid burnable poison rods, containing burnable poison in a region upwardfrom upper ends of said short-length fuel rods and adjacent to saidshort-length fuel rods in said four directions of front and rear andright and left is one-half or less the total number of said burnablepoison rods; and at a transverse cross-section of the region upward fromupper ends of said short-length fuel rods, the amount of burnable poisoncontained in a polygonal region, whose vertexes are located at centersof those of said short-length fuel rods arranged at the outermost layeris smaller than the amount of burnable poison outside said polygonalregion; wherein fuel rods being adjacent to said short-length fuel rodsin said four directions of front and rear and right and left and beingarranged at the outermost periphery are adjacent to one of said burnablepoison rods in said four directions of front and rear and right andleft.
 2. A fuel assembly comprising: a plurality of fuel rods includinga plurality of short-length fuel rods shorter in length than others ofsaid fuel rods and a plurality of long-length fuel rods, saidlong-length fuel rods having burnable poison rods with burnable poisonand fissionable material and fissionable material rods with no burnablepoison and fissionable material, said plurality of fuel rods beingarranged in a square grid pattern of 9-columns/9-rows or more; and atleast one neutron moderator rod; wherein each of said short-length fuelrods is arranged at a position other than in 3-columns/3-rows regions atfour corner portions in such a manner as not to be simultaneouslyadjacent to said at least one neutron moderator rod and others of saidshort-length fuel rods in the four directions of front and rear andright and left; said burnable poison rods are arranged at positionsexcluding the outermost periphery; the number of those, of said burnablepoison rods, containing burnable poison in a region upward from upperends of said short-length fuel rods and adjacent to said short-lengthfuel rods in said four directions of front and rear and right and leftis one-half or less the total number of said burnable poison rods; andat a transverse cross-section of the region upward from upper ends ofsaid short-length fuel rods, the amount of burnable poison contained ina polygonal region, whose vertexes are located at centers of those ofsaid short-length fuel rods arranged at the outermost layer is smallerthan the amount of burnable poison outside said polygonal region;wherein 74 pieces of said fuel rods including 6 pieces of saidshort-length fuel rods and 12 pieces or more of said burnable poisonrods are arranged in a square grid pattern of 9-columns/9-rows, and twopieces of large-sized water rods are arranged in a region containing 7grid points at a central portion of said fuel assembly.
 3. A fuelassembly comprising: a plurality of fuel rods including a plurality ofshort-length fuel rods shorter in length than others of said fuel rodsand a plurality of long-length fuel rods, said long-length fuel rodshaving burnable poison rods with burnable poison and fissionablematerial and fissionable material rods with no burnable poison andfissionable material, said plurality of fuel rods being arranged in asquare grid pattern of 9-columns/9-rows or more; and at least oneneutron moderator rod; wherein each of said short-length fuel rods isarranged at a position other than in 3-columns/3-rows regions at fourcorner portions in such a manner as not to be simultaneously adjacent tosaid at least one neutron moderator rod and others of said short-lengthfuel rods in the four directions of front and rear and right and left;said burnable poison rods are arranged at positions excluding theoutermost periphery; the number of those, of said burnable poison rods,containing burnable poison in a region upward from upper ends of saidshort-length fuel rods and adjacent to said short-length fuel rods insaid four directions of front and rear and right and left is one-half orless the total number of said burnable poison rods; and at a transversecross-section of the region upward from upper ends of said short-lengthfuel rods, the amount of burnable poison contained in a polygonalregion, whose vertexes are located at centers of those of saidshort-length fuel rods arranged at the outermost layer is smaller thanthe amount of burnable poison outside said polygonal region; wherein 74pieces of said fuel rods including 6 pieces of said short-length fuelrods and 12 pieces or more of said burnable poison rods are arranged ina square grid pattern of 9-columns/9-rows, and two pieces of large-sizedwater rods are arranged in a region containing 7 grid points at acentral portion of said fuel assembly; wherein four pieces of saidshort-length fuel rods are arranged at each center of four outermostperipheral sides, and two pieces of said short-length fuel rods arearranged at positions adjacent to said water rods.
 4. A fuel assemblycomprising: a plurality of fuel rods including a plurality of first fuelrods shorter in length than others of said fuel rods and a plurality ofsecond fuel rods containing burnable poison, said plurality of fuel rodsbeing arranged in a square grid pattern of 9-columns/9-rows or more; andat least one neutron moderator rod; wherein said first fuel rods arearranged at positions other than in 3-columns/3-rows regions at fourcorner portions, and said first fuel rods are arranged at each side ofthe outermost periphery; said second fuel rods are arranged at positionsexcluding the outermost periphery; the number of those, of said secondfuel rods, adjacent to said first fuel rods in the four directions offront and rear and right and left is one-half or less the total numberof said second fuel rods; and at a transverse cross-section of theregion upward from upper ends of said first fuel rods, the amount ofburnable poison contained in a polygonal region, whose vertexes arelocated at centers of those of said first fuel rods arranged at theoutermost layer, is smaller than the amount of burnable poison outsidesaid polygonal region, wherein said first fuel rods arranged at theoutermost layer have no burnable poison.
 5. A fuel assembly comprising:a plurality of fuel rods including a plurality of first fuel rodsshorter in length than others of said fuel rods and a plurality ofsecond fuel rods containing burnable poison, said plurality of fuel rodsbeing arranged in a square grid pattern of 9-columns/9-rows or more; anda neutron moderator rod; wherein said first fuel rods are arranged atpositions other than in 3-columns/3-rows regions at four cornerportions, and said first fuel rods are arranged at each side of theoutermost periphery; said second fuel rods are arranged at positionsexcluding the outermost periphery; and most of fuel rods being adjacentto said first fuel rods in the four directions of front and rear andright and left and containing no burnable poison have the highestaverage enrichment of a fissile in the region upward from upper ends ofsaid first fuel rods, wherein said first fuel rods arranged at theoutermost layer have no burnable poison.